1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to a coupled resonator filter and a fabrication method thereof, and more particularly, to a coupled resonator filter on which a plurality of materials are sequentially stacked and patterned to sequentially stack a first electrode, a first piezoelectric layer, a second electrode, an insulating layer, a third electrode, a second piezoelectric layer, and a fourth electrode, and a fabrication method thereof.
2. Description of the Related Art
Demands for compact, light filters used in mobile communication devices represented by portable phones have been rapidly increased with the rapid popularization of the mobile communication devices. Film bulk acoustic resonators (FBARs) have been known as powerful members for constituting such compact, light filters. FBARs can be produced at a minimum cost and realized as being compact. The FBARs can realize a high quality factor Q as a major characteristic of filters and be used in a micro-frequency band.
In general, an FBAR has a structure in which a first electrode, a piezoelectric layer, and a second electrode are sequentially stacked. To operate the FBAR, electric energy is applied to the first and second electrodes to induce an electric field inside the piezoelectric layer and produce a piezoelectric phenomenon in the piezoelectric layer due to the electric field so as to generate a bulk acoustic wave. As a result, a resonance is produced.
A plurality of FBARs are combined into a ladder type or lattice type filter so as to be used as a filter. However, the ladder type or lattice type filter cannot perform a transformation operation between an unbalanced signal and a balanced signal. Thus, the ladder type or lattice type filter must further include a balun structure to perform the transformation operation.
To solve this problem, there has been developed a coupled resonator filter which is capable of outputting both a balanced signal and an unbalanced signal when an unbalanced signal is input. The coupled resonator filter is basically fabricated by vertically stacking two FBARs.
U.S. Pat. No. 6,670,866 discloses a coupled resonator filter. The disclosed coupled resonator filter is isolated from a substrate using a bragg reflector. Materials having great difference in elastic impedance are alternately stacked to form the bragg reflector. Thus, a bulk acoustic wave (BAW) having passed through a piezoelectric layer inside the coupled resonator filter is not transmitted toward the substrate but reflected by the bragg reflector so as to produce an efficient resonance.
According to U.S. Pat. No. 6,670,866, it is difficult to fabricate the bragg reflector formed of four or more layers having uniform thicknesses. As a result, time and cost for fabricating the coupled resonator filter are increased.